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4. Discussion

4.1. Source structure

The inclusion of the quasar 1422+202 in the CSS class of source by Mantovani et al. (1992) was mainly based on its relatively small linear size of 43 kpc (H0= tex2html_wrap_inline1282 tex2html_wrap_inline1284) and on its overall spectral index in the radio band. Since it was clear we were dealing with a compact object the suggestion of 1422+202 as a candidate CSS was not considered a compelling choice in a period where the understanding of the CSSs class of object was growing. Moreover, the source exhibits a radio power of tex2html_wrap_inline1286 W/Hz at 178 MHz which is comparable to the radio power found for the CSSs selected from the 3CR catalogue. On the other hand the linear size of 1422+202, exceeds the selection criteria (linear size tex2html_wrap_inline1288 kpc) set by Fanti et al. (1995) in a paper in which the concept of a connection between CSSs and larger sized radio sources is introduced: the CSSs represent the early stage in an evolutionary sequence. Following the line of their work, we can think of 1422+202 as a Medium-size Object having an asymmetric structure.

The core fractional luminosity is tex2html_wrap_inline1290, a value consistent with the median value found for both CSSs and large size radio quasars. Relativistic boosting might be present in the core. However, only a weak and short jet coming out from the core is detected in the VLAtex2html_wrap_inline1244 observation and it is not seen in the VLBI image, suggesting that the sub-arcsecond jet is not boosted. So, if we assume the average velocity of growth of tex2html_wrap_inline1294 for Medium-size Symmetric Object as in Fanti et al. (1995) the time required to grow for 1422+202 will be tex2html_wrap_inline1296 years, larger than the CSSs age in the youth scenario.

The jet dominates the arcsecond scale structure of 1422+202 at 5 and 8.4 GHz. The major axis Position Angle of the various components along the thin collimated jet indicates a possible helical shape. The origin of such a structure might be intrinsic to the core region or due to physical processes in the jet. Bends along the jet might be caused by interaction between the jet flow and dense gas clouds or could be the result of interference shocks in the plasma going down the jet (Hardee 1990). Such interactions can also brighten the emission. However, if the bends are formed by the former model we should see the effect of the interaction on the polarized emission. There is just a marginal indication of a difference in depolarization between the two sides of the source and the Faraday rotation of individual components is negligible going from 8.4 GHz to 5 GHz (Mantovani et al., in preparation). This lack also excludes frustration as an explanation for the asymmetric structure. The asymmetry is then the result of an intrinsic process.

The spectral index in 1422+220 is everywhere steeper than 0.4. The core area has a spectrum which is flatter than the spectrum along the jet and at both the hot spot regions. The core shows a Giga-Hz-Peaked spectral index with the peak of emission at about 4 GHz. On the other hand, the resolution achieved by mapping the source with the VLA, did not allow to isolate the hot spots from the surrounding source structure. Their spectral indices are quite steep (tex2html_wrap_inline1298). Similar values were also found by Lonsdale tex2html_wrap_inline1300 Barthel (1984) for 3C205 and by Carilli et al. (1992) for the high frequency part of the spectral index in both the hot spots of Cygnus A.

4.2. Low frequency variability

The variability reported for 1422+202 at low frequency in monitoring observations (Bondi et al. 1994) has probably an origin extrinsic to the source. Refractive scintillation (Rickett 1986) can produce a variation of a few percent in flux density at frequencies < 1 GHz in sources with compact bright structures similar to the hot spot found in 1422+202. The source has a scintillation index of 0.018 (Bondi et al. 1994). Since the source is not point like, a compact component responsible for the refractive scintillation should be present inside its structure. Assuming a component which is tex2html_wrap_inline1304 in flux density at 408 MHz (the total mean flux of the source is tex2html_wrap_inline10505Jy), the scintillation index (rms flux density variation mean flux density ratio) will be 0.04-0.03. The refractive scintillation theory (Rickett 1986) suggests that there is a correlation between the scintillation index, the galactic latitude and the source size. If we take a gaussian component as a simple case, from Fig. 4 of Spangler et al. (1993) we can derive a component size tex2html_wrap_inline105030 mas. The south hot spot has a component detected in our VLBI 18 cm observations which has a spectral index that extrapolated gives tex2html_wrap_inline1304 of flux density at 408 MHz. This might explain the low frequency variability detected for 1422+202 as due to an extrinsic mechanism, a point of view which is supported by the lack of flux density variation at frequencies >2GHz. However, this hot spot would have to have unusual properties among hot spots found in radio galaxies, which usually show angular sizes of the order of 200 mas. A VLBI observation at low frequency is needed to confirm the existence of such a component.

4.3. Phase referencing technique

The successful application of the phase referencing technique to the source 1422+202 allowed the detection of two components with absolute positions, relative to OQ208, corresponding to the core region and to the south hot spot region of the radio source. This experiment was not designed as a pure astrometric one, with short duty cycle in switching between the reference and target source. Despite the fact that the reference source was observed for just three 13 minutes scans all along the 11 hours tracking of the target source, this experiment shows that the EVN can work as a phase stable instrument at 1.6 GHz for separation up to tex2html_wrap_inline1318 between the sources and the reference calibrator.

The positional accuracy of tex2html_wrap_inline1320 mas is much worse than that usually achieved in astrometric experiments which is at tex2html_wrap_inline1322 arcsec level. However, the method can be extended to the cases where it is difficult to design a specific astrometric experiment.


F.M. thanks the Director, Onsala Space Observatory and L.B.B thanks the Director, Istituto di Radioastronomia, for their hospitality during periods when parts of the work presented here were done. We also like to thanks the correlator staff of the Max-Planck-Institut für Radioastronomie. The Onsala Space Observatory at the Chalmers University of Technology is the Swedish National Facility for Radioastronomy. The VLBI project at Onsala is supported by the Swedish National Science Foundation under grant F-FU 4876-302. Fredrik.T.Rantakyrö acknowledges support for his research by the European Union under contract ERBCHGECT920011. The National Radio Astronomy is operated by Associated Universities Incorporated under cooperative agreement with the National Science Foundation. AIPS is the NRAO's Astronomical Image Processing System.

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